On Wed, Jul 29, 2015 at 05:08:55PM +0200, Michal Hocko wrote: > On Wed 29-07-15 17:45:39, Vladimir Davydov wrote: > > On Wed, Jul 29, 2015 at 07:12:13AM -0700, Michel Lespinasse wrote: > > > On Wed, Jul 29, 2015 at 6:59 AM, Vladimir Davydov <vdavydov@xxxxxxxxxxxxx> > > > wrote: > > > >> I guess the primary reason to rely on the pfn rather than the LRU walk, > > > >> which would be more targeted (especially for memcg cases), is that we > > > >> cannot hold lru lock for the whole LRU walk and we cannot continue > > > >> walking after the lock is dropped. Maybe we can try to address that > > > >> instead? I do not think this is easy to achieve but have you considered > > > >> that as an option? > > > > > > > > Yes, I have, and I've come to a conclusion it's not doable, because LRU > > > > lists can be constantly rotating at an arbitrary rate. If you have an > > > > idea in mind how this could be done, please share. > > > > > > > > Speaking of LRU-vs-PFN walk, iterating over PFNs has its own advantages: > > > > - You can distribute a walk in time to avoid CPU bursts. > > > > - You are free to parallelize the scanner as you wish to decrease the > > > > scan time. > > > > > > There is a third way: one could go through every MM in the system and scan > > > their page tables. Doing things that way turns out to be generally faster > > > than scanning by physical address, because you don't have to go through > > > RMAP for every page. But, you end up needing to take the mmap_sem lock of > > > every MM (in turn) while scanning them, and that degrades quickly under > > > memory load, which is exactly when you most need this feature. So, scan by > > > address is still what we use here. > > > > Page table scan approach has the inherent problem - it ignores unmapped > > page cache. If a workload does a lot of read/write or map-access-unmap > > operations, we won't be able to even roughly estimate its wss. > > That page cache is trivially reclaimable if it is clean. If it needs > writeback then it is non-idle only until the next writeback. So why does > it matter for the estimation? Because it might be a part of a workload's working set, in which case evicting it will make the workload lag. Thanks, Vladimir -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
